One of the most rapidly developing areas of electronic products research is that of optical fibers. Useful in optical scanning devices, holography, communication, and surgery, optical fibers are regarded as an extremely important part of our future.
A significant problem in utilizing optical fibers in many of the current and proposed applications is the need to coat the fibers with a material capable of providing moisture resistance, high temperature resistance, and abrasion resistance without a loss in flexibility, or marked increase in size or cost.
Amorphous silicon (hereinafter a--Si) films have been the subject of intense research for various applications in electronic industries. They have been, however, least explored with respect to optical fiber coating applications and only a few patents have appeared in the literature.
Thus, there is a need for a process for producing silicon-containing coatings for optical fibers at low equipment cost and with acceptable film characteristics and uniformity. Inorganic type coating materials, such as a--Si, for example, are highly desirable for such applications compared to organic polymer coatings, since the former is expected to provide excellent protection to optical fibers from moisture-related static and dynamic fatigue. The optical fiber coating of preference is one which has a refractive index significantly different from, and higher than, that of the optical fiber itself so as to promote diffraction and scattering of any light which may have escaped the fiber while being transmitted. For many applications utilizing optical fibers, it is most desirable to have a coating on the fibers which is a poor electrical conductor or an electrical non-conductor. Thus, non-metallic coatings are frequently preferred as coatings for optical fibers in these applications. Silicon-containing polymeric coatings can be deposited which are excellent in this regard as a result of their non-conductive character relative to metals for electromagnetic interference and electromagnetic pulse resistance.
The energy transmitted through the optical fiber is electromagnetic radiation which may include wavelengths outside the visible range. For purposes of this application, the electromagnetic radiation will be referred to as light. The light or energy which escapes from the core fiber does not contribute to the meaningful information transmission, and there is a need to prevent it from returning to the fiber core because it would be out of phase with the main signal. Preventing the light which has escaped from the fiber core from again entering the fiber core is usually accomplished by coating the fiber with a protective coating which has a higher refractive index than that of the fiber. This higher refractive index coating refracts or disperses the unwanted light away from the fiber core. Such coatings are often referred to as optical fiber coatings or wave guide coatings. Silicon coatings have very high refractive indices, on the order of 4.5 and thus are attractive as optical fiber coatings or wave guide coatings. "Wave guide" is another name applied to a device for conducting light and may be on a substrate other than a free standing fiber.
Chemical vapor deposition (CVD) is one of the most widely used deposition processes to coat surfaces. Conventional CVD process is based on thermochemical reactions such as thermal decomposition, chemical reduction, displacement and disproportionation reactions. CVD reaction products find applications in a wide variety of fields; providing hard coatings on cutting tools, protecting surfaces against wear, erosion, corrosion, high temperature oxidation, high temperature diffusion, solid state electronic devices, preparation of fibers made of composite materials, and hermetic coatings.
A number of possible processes have been disclosed up to now for depositing films. For instance, for producing films of amorphous silicon deposit, there have been tried the vacuum deposition process, plasma CVD process, CVD process, reactive sputtering process, ion plating process and photo-CVD process, etc. Generally, the plasma CVD process is industrialized and widely used for this purpose.
However, deposited films of amorphous silicon still admit of improvements in overall characteristics including electrical and optical properties, various characteristics of fatigue due to repeated uses or to environmental use conditions. In addition, productivity of depositing silicon-containing films presents problems in product uniformity, reproducibility, and mass-production.
The conventional plasma CVD process is regarded at the present time as the best method for the purpose of obtaining amorphous silicon films which have such electrical and optical properties so as to fulfill various application purposes. However, the conventional CVD process requires a high operational temperature and therefore is somewhat limited in applications and substrates.
Silicon-containing polymeric materials of silicon and hydrogen (hereafter referred to as a--SiH) have emerged as a new class of coatings in recent years. Such materials are described, for example, in D. Carlson, U.S. Pat. No. 4,064,521, issued on Dec. 20, 1976. The materials are generated as thin films from the decomposition of silane (SiH.sub.4) in electrical discharges or, less frequently, from the thermal decomposition of silane or higher hydrogen-containing silanes (e.g., Si.sub.2 H.sub.6, Si.sub.3 H.sub.8, etc.) as described in a PCT patent application by A. MacDiarmid and Z. Kiss published as International Publication No. WO82/03069 dated Sep. 16, 1982.
U.S. Pat. No. 4,459,163, issued Jul. 10, 1984 to MacDiarmid and Kiss, teaches the preparation of amorphous semiconductor material that is suitable for use in a wide variety of devices by the pyrolytic decomposition of one or more gaseous phase polysemiconductanes, including polysilanes and polygermanes. However, U.S. Pat. No. 4,459,163 is directed toward the formation of semiconductor material and not optical fiber coating material.
U.S. Pat. No. 4,374,182, issued Feb. 15, 1983 to Gaul et. al., discloses decomposing halogenated polysilanes at an elevated temperature. Gaul et al., however, is limited to the pyrolysis of polychlorosilanes.
U.S. Pat. Nos. 2,606,811, issued on Aug. 12, 1952 to Wagner and 4,079,071, issued on Mar. 14, 1978 to Neale, addressed the decomposition at elevated temperatures of halogenated disilanes. However, these patents are distinguished from the instant invention because both are directed toward the hydrogenation of di- and polysilanes for the formation of silanes, and more specifically, monosilanes. Neither Wagner nor Neale teach the formation of optical fiber coatings produced by the vapor phase deposition of silicon-containing coatings from the thermal decomposition of halosilanes.
United Kingdom Patent No. 2,148,328, issued to M. Hirooka, et al., on May 30, 1985, teaches the decomposition of various silanes, including monomeric halosilanes (Si.sub.n X.sub.2n+1, where n=1), cyclic polymeric halosilanes (SiX.sub.2).sub.n, where n is greater than or equal to 3, di- and polysilanes such as Si.sub.n HX.sub.2n+1 and Si.sub.n H.sub.2 X.sub.2n. These materials are decomposed via electric discharge, or photolysis, or high temperature or catalytically and, unlike the instant invention, mixed with a requisite second stream consisting of a vapor phase material selected from the group consisting of H.sub.2, SiH.sub.4, SiH.sub.3 Br, or SiH.sub.3 I wherein the second stream has also been decomposed. The obvious disadvantage of such prior art, one which clearly distinguishes it from the instant invention, is the necessity of having two materials to decompose.
U.S. Pat. No. 4,372,648, issued Feb. 8, 1983, to Black describes an optical fiber in which there is a multitude of coatings of differing refractive index materials arranged in a specific order to provide an optical fiber which can be used as a secure communications transmissions line. The particular order of the refractive index materials in the fiber provides the ability to determine if the fiber has been tampered with to obtain the information being sent.
U.S. Pat. No. 4,512,629, issued Apr. 23, 1985 to Hanson et al. teaches the use of silicon and carbon containing coatings for the hermetic protection of optical fibers. Hanson et al., however, describes the Si--C bond in the coating as essential to the production of a hermetic corrosion resistant coating. The coatings of the instant invention do not require carbon, and in fact, organic groups in general and hydrogen are deleterious to the coatings claimed herein.
Organic coatings designed for the protection of optical fibers should have a high refractive index which effectively prevents the errant light from returning to the fiber core. Organic coatings for optical fibers, however, are used predominantly for cushioning to protect the fibers from mechanical or physical damage. In addition, organic coatings and polydimethylsiloxane coatings do not provide hermetic coatings capable of preventing moisture penetration. Furthermore, manufacturers of optical fiber cables try to minimize the use of materials which are known to generate hydrogen when the cables are in use. The desire to avoid hydrogen-liberating materials is due to the diffusion of hydrogen into the optical fiber core which leads to attenuation of signals. Since many thermally cured siloxane coatings release measurable amounts of hydrogen when exposed to moisture and elevated temperatures, these materials have a significant drawback.
Japanese Patent O.P.I. 48,001/83, published Mar. 19, 1983, by Aoki et al., described a method of coating a glass fiber with amorphous silicon by glow discharge.
U.S. Pat. No. 4,002,512, issued Jan. 11, 1977 to Lim, teaches a method of depositing a layer comprising SiO.sub.2 on a surface of a substrate at a rate which is temperature independent whereby the method combines dichlorosilane with an oxidizing gas, such as oxygen, carbon dioxide, nitrous oxide, or water to form the SiO.sub.2.
U.S. Pat. No. 4,149,867, issued Apr. 17, 1979 to Takeshi, et al., teaches a method for forming a SiO.sub.2 "soot" on the optical fiber surface by CVD oxidizing certain starting materials.
Another method for depositing a silicon oxide coating on an optical fiber is that taught in U.S. Pat. No. 3,957,474, issued May 18, 1976 to Tatsuo, et al., comprising heating by means of a carbon dioxide laser, a mixture of oxygen, a dopant compound, and pure silicon tetrachloride vapor so as to deposit silicon oxide and an oxide of the dopant compound on a mandrel and to form a fiber by fusing the oxides.